Tuesday, November 23, 2010

After the Snow Falls...

The snow and wind have finally stopped here in Seattle. In fact, today looks like a gorgeous day outside--I've never seen the sky this blue out here, as a matter of fact. Bright, sunny, though still very chilly. Temperatures are only in the mid-20s for much of the Puget Sound region.

So how bad was it? I measured the snow accumulation outside my house.

That's about four inches of snow outside my place. (Note how I used an official NOAA Office of Education ruler to make this measurement--therefore this must be a very official measurement...regardless of there being a fence nearby...). How did things pan out last night?

Fig 1 -- 24 hour meteogram from the top of the UW Atmospheric Sciences building as of 2047Z, Nov 23 2010.

The most recent time on the above meteogram is to the right. I described meteorgrams in a previous blog post, but remember that meteorgrams are just several time series charts of how a particular meteorological variable has changed over time at a certain place. This one starts at the left early yesterday afternoon and this afternoon is on the right. In the first panel, we see that winds all night were pretty strong--in the 10-20 knot range with frequent gusts over 20 knots. Winds were also rather steady out of the north, as we can see in the second panel. Our temperatures hit a low of near 20 degrees last night, which, combined with the strong winds has caused a lot of freezing on the roadways and making travel treacherous. (In fact, the University of Washington cancelled classes today, so I get an unexpected day off...).

Snowfall amounts are difficult to measure from an automated station, and the one on the roof of the atmospheric sciences building isn't really equipped for that. However, some snow apparently did melt and fall into the rain gauge (they may have a heated rain gauge, actually...I'll have to check on that...). However, I suspect this is just random snow melting into the rain gauge. I'm guessing this because the precipitation amounts (the second panel from the bottom) have continued to increase slowly during the day today even though there definitely has been no new snowfall. So, I wouldn't trust the precipitation measurements from this plot.

As this cyclone has been departing, you can see in the pressure plot (the third from the bottom) that our pressures have been steadily rising. This points to high pressure and widespread subsidence building in, which suppresses vertical motion in the atmosphere. This is why the skies are so clear--it's more difficult to form clouds without some kind of lifting mechanism. You can see for yourself how clear it is in the last panel of the meteogram, which is showing incoming solar radiation. On a completely clear day you'd expect to see a nice sinusoidal curve of the shaded region which shows increasing solar radiation as the sun climbs higher in the sky in the morning and decreasing solar radiation as the sun goes down in the evening. You can see that so far today we have a nearly perfect sinusoidal curve, meaning very clear skies. Compare that to yesterday on the left hand side of the plot where all the thick clouds kept us from getting much solar radiation at all. Remember--clouds don't block out ALL the solar radiation--otherwise it would be completely dark on a cloudy day. Some still gets through--though not much.

Another view of the clear weather over Washington:

Fig 2 -- Visible satellite image of Washington state from GOES-W for 2045Z, Nov 23, 2010. From the College of DuPage website.

There are still some low clouds over much of the eastern part of Washington and out over the Pacific. But in general--all the white and lighter grays you see across the Cascades, the Olympics and the Puget Sound lowlands--that's all snow. It didn't miss anyone, so it seems. The bright spot southeast of Seattle, by the way, is the snow cap on Mount Rainier...

The low pressure center this afternoon had already moved to somewhere in the southern Idaho/northern Utah area (it's difficult to get an exact fix because of the difficulties of dealing with pressure in the mountains--more on that in another blog). This storm has lost none of its potency--my friend Joe in Salt Lake City has been talking about the blizzard warnings going up for that area today. Not the best environment to being trying to fly in.

However, I think this system is only going to deepen as it moves out over the Rockies and into the plains and midwest. Models also agree with this conclusion, but how can we see that now?

Based on a model-derived upper air analysis for this afternoon, we can see the deep 500 mb trough over northern Idaho. Remember that troughs tend to be associated with colder air (cooling the air in a column compresses it, causing the depth of the column to shrink--and therefore the heights to fall). But in a more obvious way of looking at it, just look at how cold we got in Seattle as this trough came through. Anyhow, this trough is moving slightly south and east and bringing its cold air with it. As this cold air moves south and east, it's going to encounter the much warmer air over Texas and the southwest. This is going to strengthen the temperature gradient at lower levels as the trough heads in that direction. Remember our thermal wind argument--what happens to the winds aloft as the temperature gradient below increases?

You can see in this ECMWF model projection how much winds have increased to the southeast of the trough--where the temperature gradients will probably be the strongest. Another wonderful example of the thermal wind in action (at least according to the models). We now have a jet streak that is somewhat cyclonically-curved with an exit region over the high plains. We know what tends to happen under the exit regions of cyclonically-curved jet streaks--surface cyclogenesis. Aiding this spin-up will be the fact that the low pressure center will be coming off of the high terrain of the Rockies to the lower terrain of the plains. Without getting into the very technical details of this, in fluid dynamics if you stretch a rotating column of fluid, the column will rotate even faster. As our surface cyclone moves from higher to lower terrain, the depth of the column (in our case, the distance from the ground to the tropopause) will increase as the terrain drops down--stretching the column. This means that the circulation around the low pressure center should increase (further strengthening the cyclone).

This all adds up to what could be a powerful storm for the upper midwest. We'll be watching in the days to come to see just how bad it gets.

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Completed graduate school at the University of Washington, now a postdoctoral researcher at NCAR in Boulder. The thoughts and opinions expressed on this blog are solely those of the author and are do not necessarily represent the positions of NCAR.
Email me at lukemweather@gmail.com.
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